SYSTEMD_UNIT_PATH | How to provide *additional* search paths for systemd unit files.

gistfile1.txt

SYSTEMD_UNIT_PATH note 2023.10.21

How to provide *additional* search paths for systemd unit files.

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SYSTEMD_UNIT_PATH can be used to provide additional search paths for systemd unit files - IF the variable ends with a colon.
SYSTEMD_UNIT_PATH MUST be available before systemd is started.
So the only way I have found to do this is to set it as "kernel parameters".

I have so far made it work in 3 contexts:
    1. In hardware via GRUB
    2. In LXD
    3. In a Proxmox container

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First make the custom directory:
(root user is assumed for all operations)
In my case:
    mkdir /opt/system.custom

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1. In hardware via GRUB

    1.1. Edit /etc/default/grub
        Add the following line:
            GRUB_CMDLINE_LINUX="<other stuff> SYSTEMD_UNIT_PATH='/opt/system.custom:'"
        Save the file.

    1.2. Update GRUB
        -[debian] update-grub
        -[RHEL] grub2-mkconfig -o /boot/grub2/grub.cfg
        reboot

    Or if grub2-mkconfig is not desirable, edit directly
    /boot/grub2/grub.cfg:
      set kernelopts="<other stuff> SYSTEMD_UNIT_PATH='/opt/sofa/system.custom:'"

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2. In LXD

lxc config edit <name eg. debsid>
    config:
      environment.SYSTEMD_UNIT_PATH: '/opt/system.custom:'

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3. In a Proxmox container

Edit the container config file:
    eg: /etc/pve/nodes/blambox/lxc/12002005.conf
    Add the following line:
        lxc.environment: SYSTEMD_UNIT_PATH=/opt/system.custom:

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To test that the custom search path is in effect, run:
    systemctl show --property=UnitPath

I also put a dummy unit-file "aabbcc.service" in /opt/system.custom
Then I ran:
    systemctl daemon-reload OR rebooted
    systemctl cat aabbcc.service

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Documentation:
    https://www.freedesktop.org/software/systemd/man/latest/systemd.unit.html#Unit%20File%20Load%20Path

    Unit File Load Path
    Unit files are loaded from a set of paths determined during compilation, described in the two tables below. Unit files found in directories listed earlier override files with the same name in directories lower in the list.

    When the variable $SYSTEMD_UNIT_PATH is set, the contents of this variable overrides the unit load path. If $SYSTEMD_UNIT_PATH ends with an empty component (":"), the usual unit load path will be appended to the contents of the variable.

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